Many states allow FTF sale as they don't have a law saying its illegal to purchase under X age. The law typically defines what a concealable deadly weapon is (knives, pistol, sbr's, etc) and its age of posession. Since the age is typically defined as 18 for posession and they omit to bring purchasing of the weapon into play, the age for purchase is simply defined as 18. However, some people don't understand that laws exist to make an action a crime and not make an action lawful-- so they are skiddish of what they consider a "grey area."
However, Federal law specifically states dealers (FFL) can only sell pistols or pistol caliber ammunition to persons of age 21 or older. FEDERAL LAW PROVIDES NO MINIMUM AGE FOR FACE TO FACE TRANSFERS OF ANY KIND. The definition is worded specifically as guidance for federalyl licensed dealers. However, I do believe there are minimum age for posession in federal law (something like 18 for handguns). This makes finding ammo for a handgun at 18 a real bitch, I know from personal experience. Since the definition of federal law is "pistol caliber," it means even suggesting the ammo is for a marlin camp carbine wouldn't float.
Add NV to your list.